The present invention relates generally to the field of high performance plastic bonded explosive (PBX) compositions for use in explosive warheads for military weapons systems and comparable applications. The improved PBX compositions of the present invention have been found to demonstrate higher energy density and increased penetration power and impetus while maintaining material safety and handling characteristics comparable to conventional PBX compositions.
It is generally known in the art of military explosives to formulate plastic bonded explosive (PBX) compositions consisting of three principal components: (1) an oxidizer; (2) a binder; and, (3) a plasticizer. PBX formulations consisting of an oxidizer and a thermoplastic elastomeric (TPE) binder are also common. Based on their chemical properties, conventional binders are commonly characterized as either xe2x80x9cenergeticxe2x80x9d or xe2x80x9cinertxe2x80x9d. Conventional oxidizers, whether energetic or inert, are termed xe2x80x9coxidizers.xe2x80x9d
In general, conventional PBX compositions are prepared by adding the two or three ingredients, as solid powders or small particles, and in certain predetermined proportions, to the thermally jacketed container of a heated mixing device while maintaining a temperature inside the jacketed container of about 120xc2x0 F. to 140xc2x0 F., and blending the ingredients until the mixture is consistent and homogeneous. The thoroughly blended PBX mix is subsequently pressed and/or extruded into billets of the desired size for packing into warheads.
Two familiar oxidizers which can be used in conventional PBX compositions are: (1) HMX, a type of oxidizer standing for xe2x80x9cHigh-Melting Point Explosive,xe2x80x9d is also known as Octogen and is chemically known as cyclotetramethylenetetranitramine; and (2) RDX, a type of oxidizer standing for xe2x80x9cRoyal Demolition Explosive,xe2x80x9d is also known as Cyclonite or Hexogen, and is chemically known as cyclotrimethylenetrinitramine. Descriptions of the chemical compositions and properties of HMX and RDX can be found in the following publication: xe2x80x9cEngineering Design Handbook: Explosives Series Properties of Explosives of Military Interest,xe2x80x9d Army Materials Command, National Technical Information Services, U.S. Department of Commerce (January, 1971). While HMX has commonly been used in explosive compositions, RDX is more commonly used in propellants.
Some known xe2x80x9cenergeticxe2x80x9d binders used in PBX compositions include nitrocellulose, nitrostarch, polyvinylnitrate, and nitropolyurethanes. Some known inert binders used in PBX compositions include celluloseacetate (CA), celluloseacetate butyrate (CAB), hydroxy-terminated polybutadiene (HTPB), and polyurethanes. Some conventional energetic plasticizers are: BDNPF (Bis-2,2-Dinitropropyl Fumarate); NG (Nitroglycerin); Methyl/Ethyl Nena; Butyl Nena; MTN/DEGDN (Metriol trinitrate/Diethylene Glycol Dinitrate); and DEGDN (Diethylene Glycol Dinitrate). Some conventional inert plasticizers include: TA (Triacetin); DEP (DiethylPhathalate); and DBP (DibutylPhathalate).
Conventional PBX compositions could be improved, however, in several respects. First, a PBX composition demonstrating a higher energy density would be more effective in a warhead on a volume equivalent basis. Second, a PBX composition which, based on its explosive characteristics, provided increased penetration power and impetus would make the warhead in which it was used a more effective weapon. Improved weapons penetration is increasingly important. This improvement is required because increased penetration would enable the military weapons to successfully strike underground enemy installations such as laboratories, airports, and chemical and biological factories. Increasingly, hostile countries, often controlled by dictatorial rulers, are resorting to such underground facilities both to avoid detection by aerial surveillance as well as to secure those facilities against attacks. The only way of disabling such facilities would be an improved warhead penetration capability. At the same time, an improved PBX composition must also demonstrate good material safety, handling and storage characteristics, comparable to or better than the characteristics of conventional PBX compositions.
The improved PBX compositions of the present invention have been found to show surprisingly higher energy density and superior explosive characteristics while equaling or bettering the material safety and handling stability of conventional PBX compositions.
Accordingly, a general object of this invention is to provide improved high performance plastic bonded explosive (PBX) compositions for use in explosive warheads for military weapons and the like.
Another general object of this invention is to provide improved PBX compositions consisting essentially of an oxidizer, a binder and a plasticizer.
Still another general object of this invention is to provide improved PBX compositions having high energy density and increased penetration power.
Still another general object of this invention is to provide improved PBX compositions while maintaining material safety and handling and storage characteristics at least comparable to conventional PBX compositions.
A specific object of this invention is to provide improved PBX compositions in which the compound identified as CL-20 is the oxidizer.
Another specific object of this invention is to provide improved PBX compositions utilizing CL-20 as the oxidizer in combination with a binder selected from the group consisting of ethylene vinyl acetate and polyisobutylene.
Another specific object of this invention is to provide improved PBX compositions utilizing CL-20 as the oxidizer and triacetin as the plasticizer in combination with a binder selected from the group consisting of ethylene vinyl acetate and polyisobutylene.
Other objects and advantages of the present invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises, but is not limited to, the PBX compositions and the related products and methods using those PBX compositions as exemplified by the following description and examples. Various modifications of the chemical compositions described herein, including the addition of minor amounts of additional ingredients which do not materially affect the basic and novel characteristics of the PBX compositions of this invention, and alternative products and methods using those compositions will be apparent to those skilled in the art, and all such modifications and variations are considered within the scope of this invention.
The present invention is generally directed to new and surprisingly more effective PBX compositions based on using a type of nitramine known as CL-20 as the oxidizer. It has now been found that PBX compositions utilizing CL-20 as the oxidizer, and particularly when the CL-20 is blended with particular combinations of binders and/or plasticizers, results in synergistic chemical combinations which demonstrate higher energy density and increased penetration power and impetus in military weapons applications. Preferred PBX compositions in accordance with the present invention consist essentially of CL-20 as the oxidizer, TA (triacetin) as an inert plasticizer or BDNPF, which is bis (2,2-dinitropropyl) fumarate, as an energetic plasticizer, and a binder selected from the group consisting of ethylene vinyl acetate and polyisobutylene.
New and more effective plastic bonded explosive (PBX) compositions are prepared in accordance with the present invention based on the use of an energetic nitramine compound known as hexanitrohexaazaisowurtzitane, and generally identified in the literature as CL-20, as the oxidizer. CL-20 was developed by China Lake Chemical Co. in the early 1990s, and is currently manufactured in small, experimental quantities by Thiokol Corp. Descriptions of the chemical structure, preparation and current uses for CL-20 appear in U.S. Pat. No. 5,693,794 titled xe2x80x9cCaged Polynitramine Compoundxe2x80x9d and in No. 5,712,511 titled xe2x80x9cPreparation of Fine Particulate CL-20,xe2x80x9d both of which are incorporated herein by reference. In general, preferred PBX compositions in accordance with the present invention consist essentially of about 80-98 wt. % CL-20 as the oxidizer blended with about 1-12 wt. % of a suitable binder and with about 1-12 wt. % of a suitable plasticizer. CL-20 based PBX compositions in accordance with the present invention also include compositions containing about 80-98 wt. % CL-20 blended with about 2-20% of a suitable binder without a plasticizer. For these PBX compositions, either the binder selected will have a lower-temperature softening point, or more heat will be required to process (press) the explosive charge, or both.
U.S. government standards for explosive compositions for weapons systems require low vulnerability for safety in storage and transport and for preserving performance and integrity over extended periods of time. The novel PBX compositions of the present invention meet or exceed these standards.
In one preferred embodiment of the present invention, the PBX compositions consist essentially of about 85-98 wt. % CL-20 as the oxidizer, blended with about 1-12 wt. % of a binder selected from the group consisting of ethylene vinyl acetate (EVA) and polyisobutylene (PIB), and with about 1-12 wt. % of a suitable plasticizer. In another preferred embodiment of the present invention, the PBX compositions consist essentially of about 85-98 wt. % CL-20 as the oxidizer, blended with about 1-12 wt. % of triacetin (TA) as the plasticizer, and with about 1-12 wt. % of a suitable binder. In a particularly preferred embodiment of the present invention, the PBX compositions consist essentially of about 85-98 wt. % CL-20 as the oxidizer, blended with about 1-12 wt. % of a binder selected from the group consisting of ethylene vinyl acetate and polyisobutylene, and with about 1-12 wt. % of triacetin as the plasticizer.
CL-20 is an energetic nitramine oxidizer which is chemically related to HMX and RDX. CL-20, however, has now been determined to demonstrate several physical and chemical properties different from HMX and RDX which make it a significantly more effective oxidizer in PBX compositions. In addition, it has been found that CL-20 may function synergistically when blended with particular binder and plasticizer components. In particular, CL-20 has been found to have a significantly higher density, heat of formation, and energy compared with HMX and RDX, as illustrated in Table I below.
At the same time, CL-20 demonstrates material safety, hazard, and processing characteristics which are generally similar to HMX and RDX, as illustrated in Table II below.
CL-20 exists as a caged, 3-dimensional structure. It exists in several polymorphs, each having a different density: xcex1-CL-20, xcex2-CL-20, and xcex5-CL-20. The most favorable CL-20 polymorph for PBX applications has been found to be the epsilon polymorph xcex5-CL-20, which shows a 7.4% higher density than HMX, significantly higher heat of formation than HMX, and similar safety and hazard characteristics to HMX. The CL-20 chemical structure is illustrated in FIG. I below. 
Although PBX compositions contain predominant amounts of the oxidizer component, it is not just the properties of the oxidizer that determine the performance and handling characteristics of the blended PBX compositions. One important factor in determining the performance of a PBX composition is the theoretical maximum density or TMD (identified by the Greek letter xcfx81) of the composition. Theoretical density is a measure of how intimately the components of a blended mixture can be mixed and how tightly the mixture can be compacted. TMD is the theoretical number obtained from theoretical calculations using thermochemical simulation programs. The actual measured value of the composition density is always lower than, but may closely approach, the TMD. The ratio of actual measured density to TMD (xc3x97100) is the xe2x80x9c% of TMDxe2x80x9d as referred to hereinafter.
With modern mixing techniques and equipment, the actual density of a PBX composition can approach 98-99% or better of the composition""s theoretical density, but of course it can never exceed theoretical density. Increased actual density of a PBX composition is highly desirable because even small increases in composition density significantly increase the explosive xe2x80x9cpunchxe2x80x9dxe2x80x94specifically, higher velocity of detonation (VOD) and increased penetration performance, as described in more detail hereinafter. Therefore, it is considered highly desirable to formulate new PBX compositions which have higher theoretical densities than conventional PBX compositions thereby yielding compositions which also have higher actual densities and which demonstrate associated superior explosive performance.
Theoretical density of a blended composition, however, is determined at least in part by the respective sizes and shapes of the molecules of the different chemical compounds which comprise the composition and the relative proportions in which the several ingredients are present. Accordingly, predicting in advance the theoretical densities of different blends of components is neither easy nor exact. In part, the novelty of the present invention resides not just in the use of CL-20 as an oxidizer in new PBX compositions, but also in the discovery that certain CL-20 polymorphs and certain binders and plasticizers function synergistically in combination with CL-20, either separately or, more preferably, together to yield new PBX compositions with higher theoretical densities than those of conventional PBX compositions, as illustrated in Table III below:
Table III shows that the CL-20 #1 formula in accordance with the present invention has a 6.7% higher theoretical density than the most comparable HMX #1 formula. The CL-20 #2 formula has a 6.5% higher theoretical density than the HMX #1 formula, and the CL-20 #3 formula has a 7.2% higher theoretical density than the HMX #1 formula. Table III also shows that the CL-20 #2 formula in accordance with the present invention has a 6.9% higher theoretical density than the most comparable HMX #2 formula. The CL-20 #1 formula has a 6.7% higher theoretical density than the HMX #1 formula, and the CL-20 #3 formula has a 7.6% higher theoretical density than the HMX #2 formula. Table III further shows that the CL-20 #3 formula in accordance with the present invention has a 7.2% higher theoretical density than the comparable HMX #3 formula. The CL-20 #1 formula has a 6.7% higher theoretical density than the HMX #3 formula, and the CL-20 #2 formula has a 6.5% higher theoretical density than the HMX #3 formula.
In testing, it has been found that some other CL-20/binder formulations which are in accordance with the present invention demonstrate even higher theoretical densities and higher ballistic potentials than the CL-20 #1, #2, and #3 formulations used for Table III above. For example, it has been found that a mixture of CL-20 blended with polyglacidyl nitride (PGN) as binder shows a theoretical density of 2.0019 and a ballistic potential of 1699161, which means that this mixture has the potential to be a highly effective PBX formulation. CL-20/PGN formulations would not generally be manufactured as explosives for warheads, however, because they are relatively sensitive to handling and shock. The more preferred CL-20 based PBX formulations in accordance with this invention combine high performance characteristics with relative insensitivity and good material handling properties.
Table IV below demonstrates the dramatic improvement in explosive performance, as measured by increased ballistic potential, that results from even small percentage increases in PBX composition densities.
Table IV above shows that a 6-7% increase in the theoretical density of a PBX composition (see Table III) is associated with an increase in the ballistic potential of the PBX composition of about 8.8-12.9% relative to a comparable HMX-based PBX composition.
Example I below illustrates the preparation of a PBX composition using CL-20 in accordance with the present invention.